The interface air/water of n-dodecanephosphonic acid solutions

 The air/water interface of the system n-dodecanephosphonic acid (H₂DP)–water was studied by surface tension, ion-selective electrodes and evaporation in an electrobalance. The combination of surface tension and ion-selective electrodes measurements enables to study the adsorption of soluble surfactants above the cmc using the Gibbs’ equation. H₂DP formed a nonideal monolayer at the air/water interface with A _molec=0.995 nm² below the cmc. Above the cmc there was a reduction in adsorption giving A _molec=6.32 nm², which remained almost constant in the explored concentration range. This adsorption reduction may be due to a change composition on micellization, or to a thermodynamic advantage of micellization on adsorption. The reduction in the evaporation rate of water was mainly due to the reduction of the water activity, caused by the presence of solutes in bulk. This is because the strong changes in the surface coverage did not have significant influence on the evaporation rate. Received: 6 January 1997 Accepted: 15 August 1997